The invention relates to a method of coating substrate surfaces with LCST polymers, in particular to a method of coating pigment particles and, very particularly, to a method of irreversibly coating particulate and non-particulate substrate surfaces in general.
LCST polymers are polymers which are soluble in a solvent at low temperatures and separate from the solution as a separate phase when the temperature is raised and the LCST (lower critical solution temperature) is reached.
A definition thereof is given, in particular, in the monograph xe2x80x9cPolymerexe2x80x9d by Hans-Georg Elias, Hxc3xcthig and Wepf-Verlag, Zug, 1996, pages 183 to 184.
Pigments are finely divided particulate materials which, when mixed with a vehicle or matrix, influence or determine the coloration thereof. For reasons of cost, pigments are used in minimum amounts. Due to interaction forces, agglomeration can take place, particularly when the pigment particles are worked into the vehicle or matrix, so that the number of particles available for imparting color is reduced. Such agglomerate is broken down usually only with considerable difficulty or in at least a sluggish manner. When it is nevertheless desirable to achieve a desired shade within an acceptable time and with justifiable expenditure, the only alternative is to raise the amount of pigment added.
On the other hand, the pigment particles, particularly particles of inorganic pigments, are of a substantially higher density than the vehicle or matrix, which is usually of an organic nature, so that sedimentation effects are observed when the pigment dispersions stand for a time.
It is an object of the present invention to treat particles such that they are stabilized as disperse particles in liquid media, such as varnishes.
In the present invention, the particles are provided with a coating of LCST polymers, this being carried out by first dissolving an LCST polymer in a solvent at a temperature below the LCST, then mixing the particles with the resulting solution, and raising the temperature of the resulting mixture to, and optionally beyond, the point at which the LCST polymer deposits onto the particle surfaces.
LCST polymers are particularly suitable for enveloping particles entirely without influencing the color of the particles themselves, since LCST polymers are completely transparent in the visible range.
Astonishingly, it is not only possible to achieve stabilization of the particles in liquid media, such as varnishes or the like, but we have also found, surprisingly, that the coloring strength of the pigments is fully retained, because coating with LCST polymers inhibits agglomeration of the fine pigment particles.
As examples of particles that can be coated by the method of the invention there may be mentioned:
Furthermore, we have found that coating the particles with LCST polymer can improve the compatibility of the particles with the vehicle or matrix, ie the particles can be rendered compatible with aqueous media or organic media regardless of the nature of the particles.
We have also found, surprisingly, that coating the particles, particularly lamellar particles of fancy pigments, with LCST polymer provides an additional protection against mechanical damage under shear loads such as occur during extrusion.
The particles are added to the solution of LCST polymer preferably in the form of a dispersion, preferably using the same solvent as that of the LCST polymer solution, whilst the temperature of the dispersion is lowered to below the LCST.
It is particularly preferred to carry out said LCST polymer coating such that the coating completely envelops the (pigment) particles.
The layer thicknesses of the LCST polymer coatings are preferably xe2x89xa720 nm, more preferably xe2x89xa725 nm, and most preferably xe2x89xa750 nm.
A particularly homogeneous thick coating is obtained on surfaces when the material to be coated is first heated in admixture with the LCST polymer solution to a temperature above the LCST, and is then cooled and reheated, once or a number of times.
The coating process is very preferably carried out such that subsequently to or during formation of the coating the LCST polymer is rendered immobile on the surface of the substrates to be coated therewith.
Several processes are suitable for said immobilization, these being such as crosslink the coating or fix it to the substrate surface. This crosslinking or fixation can be accomplished by effecting suitable modification of the polymer and/or the substrate such that chemical bonds are formed between the individual polymer chains (crosslinking) and/or between the substrate and the LCST polymer. Examples of a suitable method of modifying the LCST polymers would be the introduction of a radically crosslinkable group or of acid chloride or chloroformate groups since this would make it possible to cause reaction with OH and NH groups in the polymer (crosslinking) or with OH and NH groups on the substrate surface (fixation).
Another way of immobilizing the LCST polymer layer is of a thermodynamic nature and consists in either modifying the LCST polymer with groups which interact strongly with the substrate (due to ionic, polar, or non-polar reciprocal effects) or in achieving as low an LCST of the polymer as possible.
The literature discloses reactions in which polymers are bound to surfaces by so-called grafting-from processes. These grafting-from reactions can be initiated by the introduction of certain groups on the surface. Depending on the modification of the surface, monomers are then polymerized onto the surface radically, cationically or anionically (cf: K. Fujiki, N. Tsubokawa, Y. Sone, Polym. J. 22).
However, the polymer may also be grafted onto the surface by free-radical polymerization (cf T. Tsubokawa, A. Naitoh, J. Jpn. Soc. Colour Mater. 72 (8) (1999) 475).
A drawback of this method is that the grafting-from process demands a functionalized surface, which is not the case with all substrates and cannot always be created.
Any functionalization of the surface additionally necessary denotes an additional process step, however.
Furthermore, the subsequent polyreaction must then be carried out in the presence of the substrates, which reaction in turn calls for very specific reaction conditions, such as temperature, solvent, etc., which frequently leads to at least partial flocculation of the substrate particles. Instead of a coating of individual substrate particles there are produced coated flocculates.
The advantage of the present process resides in the general possibility of modifying surfaces of very different types with polymer layers of variable thickness. By suitably choosing LCST polymers having good active groups and an LCST as low as possible, the polymer is thermodynamically immobilized on the substrate surface so that frequently no additional crosslinking or fixation is necessary.
When selecting the LCST polymers for said thermodynamic immobilization, care is taken to ensure that the LCST temperature is below the operating temperature of the coated substrate, for example room temperature, as far as possible, and that the difference between said operating temperature and the LCST is as great as possible, ie the temperature difference is preferably xe2x89xa710xc2x0 C. and more preferably xe2x89xa715xc2x0 C. The LCST is therefore preferably  less than 15xc2x0 C. and more preferably  less than 10xc2x0 C.
Particularly suitable substrates are, in addition to pigments, fillers for plastics materials and varnishes, which fillers exist in particle form and are required to be distributed in the end product as finely and uniformly as possible. Here again, this process is suitable for improving the properties of these fillers by keeping the filler particles discrete from each other so that no agglomeration takes place and, in addition, said fillers can be simultaneously rendered compatible with the plastics matrix or the varnish medium or vehicle.
The above process is also suitable for coating non-particulate substrate surfaces with LCST polymer, this being achieved as follows:
(a) an LCST polymer is dissolved in a solvent at a temperature below the LCST;
(b) the solution produced in step (a) is brought into contact with the substrate surface of the non-particulate substrate to be coated; and
(c) the temperature is raised to or beyond the point at which deposition of LCST polymer onto the substrate surface occurs.
Another particularly important application of the present invention resides in coating semiconductor wafers as substrates with LCST polymers.
Certain areas or the whole area of the surface of said semiconductor wafer can be hydrophobized or hydrophilized with deposited LCST polymers.
Thus the process of the invention is also suitable for making masks on surfaces of semiconductor wafers by depositing LCST polymer of hydrophilic and/or hydrophobic nature onto the surface in a desired manner and immobilizing the same.
Thus the present invention relates not only to the process of the invention but also to pigments provided with a coating of an LCST polymer exhibiting the aforementioned advantages. In addition, the invention also relates to fillers exhibiting on their surface an LCST polymer coating and, in particular, semiconductor wafers having a surface coating of immobilized LCST polymer.
The pigments coated in the manner herein proposed can be used to produce binder-free pigment pastes containing a carrier medium, preferably water or an organic solvent, which by reason of their freedom from binding agents can be used universally in paints and varnishes. In this way there is no necessity to stock modified pigment pastes for paints and varnishes having different binder systems. Besides, the binder-free pigment pastes of the invention can be very simply incorporated in the accepting media (paints, varnishes, plastics materials, etc.), since by reason of the LCST coating agglomeration of the pigment particles is avoided and the mixing operation need not take this into consideration (no dispersion is necessary).
The LCST coating according to the present inventionxe2x80x94whether on particulate or non-particulate substratesxe2x80x94can serve as a protective coating for underlying coatings containing UV stabilizers, chromophores, or luminescent components.
It is also conceivably to incorporate such functional components in the LCST coating itself or, alternatively, to link them chemically to the LCST polymers, and then to deposit them onto the substrate surface.
In addition to its protective function, coating with LCST polymers may also serve to modify the particle surface, over which further functionalization of the surface can be carried out.
Within the scope of the present invention particularly suitable LCST polymers are, for example:
polyethylene oxide (PEO) derivatives
polypropylene oxide (PPO) derivatives
poly(N-isopropylacrylamide)
ethyl(hydroxyethyl)cellulose
poly(N-vinylcaprolactam) and
poly(methylvinyl ether).
These and other advantages of the invention are illustrated in greater detail below with reference to examples and the drawings.